"We are forwarding the report to our Civilian Casualty Cell for further assessment on this allegation," it said.

Iraqi warplanes have recently carried out strikes against IS in eastern Syria, while coalition aircraft have been supporting Kurdish-led fighters battling the militants. The IS fighters were Syrians and Iraqis, the Observatory said.

State news agency SANA reported the strike late Thursday, saying more than 30 civilians were killed and accusing the coalition of carrying it out.

IS fighters swept across Syria and Iraq in 2014, declaring a cross-border "caliphate" in areas they controlled.

They have since lost most of that territory to various offensives, but still retain pockets of land in Syria including in the country's vast Badiya desert and in Deir Ezzor. IS fighters have faced two separate offensives in Deir Ezzor on either side of the Euphrates River that cuts through the province.

Russia-backed regime forces have pushed back the militants on the western side of the Euphrates, while the US-backed Syrian Democratic Forces have battled them to its east.

Al-Soussa lies to the east of the Euphrates River, in a pocket of territory still held by the militants.

IS fighters have been expelled from most urban centres in Syria, but analysts say they have retained their ability to pounce from the desert.

Russian-backed government forces raised the flag in Daraa city on Thursday, but the regime still has two regions outside its control - and influential neighbours - to contend with.

To the west, it will have to retake the Quneitra province bordering the Israeli-occupied Golan Heights, before moving on to a major battle in the north near the border with Turkey.

"Bashar al-Assad sent a signal with the fall of Daraa city that nowhere in Syria that has risen up against him will remain outside his reach," said Nick Heras, an analyst at the Center for a New American Strategy.

It was in poverty-stricken Daraa that anti-Assad protests erupted in 2011, sparking an uprising that spiralled into a complex civil war.

The regime has retaken large parts of Syria with backing from its Russian ally since 2015, but few campaigns have been as quick as the one in Daraa.

A ceasefire was announced last week between opposition fighters and the regime, less than three weeks after the start of a deadly bombing campaign.

Still "it would be a mistake for the regime to let it go to its head and think that it had definitively won the war," said Karim Bitar of the Paris-based Institute of International and Strategic Affairs.

"This war in Syria is no longer exclusively Syrian, but involves many international actors who consider they have not had their last word yet."